The flaws allow the creation of accounts with elevated privileges on websites built with the popular Joomla content management system, even if account registration is disabled. They were patched in Joomla 3.6.4, released Tuesday.

Hackers didn't waste any time reverse engineering the patches to understand how the two vulnerabilities can be exploited to compromise websites, according to researchers from Web security firm Sucuri.

"Less than 24 hrs after the initial disclosure, we started to see tests and small pings on some of our honeypots trying to verify if this vulnerability was present," Sucuri's CTO, Daniel Cid, said in a blog post Friday. "In less than 36 hrs after the initial disclosure, we started to see mass exploit attempts across the web."

The attacks started Wednesday and rapidly grew in number, reaching almost 28,000 on Friday -- and that's only based on the visibility that Sucuri has into its own customer base. According to the company, virtually every Joomla website on its network has been hit.

"If you have not updated your Joomla site yet, you are likely already compromised," Cid said, adding that Joomla site owners should update immediately and should check their dashboard for newly created accounts. The site access logs can also be checked for requests that contain a "task=user.register" pattern.

Joomla is the second most popular content management system after WordPress and is used by many organizations to create both public-facing and internal websites.